Document schema validation order

[matthias-pichler]

Signed-off-by: Matthias Pichler [email protected]

Please specify parts of this PR update:

• [x] Specification
• [ ] Schema
• [ ] Examples
• [ ] Extensions
• [ ] Use Cases
• [ ] Community
• [ ] CTK
• [ ] Other

Discussion or Issue link:

#981

What this PR does:

I extended the data flow diagram with validation steps and rewrote the Data Flow section to acurately represent the execution order

Additional information: